The present invention relates to improved methods and apparatus for controlling the temperature of a product and, more particularly, to methods and apparatus to which a viscous food product is chilled with CO.sub.2.
In the course of processing certain products such as ground beef or cut chicken, it is frequently necessary to chill such products to a desired temperature. Typically, chilling is required to enable formation of such products in particular shapes, e.g. a hamburger, which may be then frozen prior to shipment to a restaurant or the like. It is known to chill ground beef, for example, in a blender by adding a refrigerant to the blender, such that the mixture of the beef and refrigerant by an impeller causes the beef to be chilled to a temperature, typically on the order of 30.degree. F. Although the actual temperature of such ground beef can be sensed by one or more temperature measuring instruments disposed in the blender, such a technique requires careful placement of these instruments and only yields point source temperature indications and many times, does not provide an indication of the overall beef temperature. Thus, although a temperature of 30.degree. F. may be indicated by one instrument, other locations in the blender may not be experiencing the same temperature and thus, the beef product may not be uniformly chilled to this temperature. Typically, a refrigerant such as CO.sub.2 is added to a blender at a predetermined flow rate for a given interval of time, which is set in response to the normal initial temperature of the beef and the total weight thereof in the blender. However, as the initial temperature of the beef and the fat content thereof may vary from average values, simply introducing CO.sub.2 into the blender for a predetermined time interval under the control of a timer does not assure that a desired final temperature of the beef will exist at the end of such time interval. For example, if the initial temperature of the beef is higher than average, the final temperature will likewise be above a desired value and, it the initial beef temperature is lower than an assumed value, the final temperature thereof will also be lower, which in turn requires that the processor must wait for the batch of chilled beef to warm up so that further processing steps, such as forming hamburger patties, or the like, may be conducted. Such delays, of course, reduce the through-put of the overall meat processing plant and also entail unnecessary consumption of CO.sub.2.
In previously known systems wherein the flow of CO.sub.2 is controlled by means of a timer, as described above, liquid CO.sub.2 is caused to flow through a control valve and is emitted into a blender from a "snow horn" or other suitable devices as solid and gaseous CO.sub.2. As the pressure in a liquid CO.sub.2 storage vessel decreases, the actual solid CO.sub.2 delivered to the blender will be reduced and consequently, in a given time interval, less refrigeration is provided to the blender than would be provided, were the liquid CO.sub.2 pressure to remain constant. Thus, simply using a timer to control the flow of CO.sub.2 to a blender frequently results in inaccurate amounts of this refrigerant being supplied, which in turn results in frequent failure to bring the beef, or other product, to a desired temperature at the end of a time interval set by the timer device. Although thermometers or other temperature indicating instruments may be utilized to check the temperature of beef at the end of a time interval in a blender, it is not unknown for operators to mistakenly leave such instruments in the batch, which error may result in the loss of an entire batch of beef product.
It is also known in the prior art to control the supply of a refrigerant which passes in heat exchange relation to a substance being refrigerated by effectively detecting the viscosity of the substance being chilled, as illustrated in U.S. Pat. No. 3,108,449. In the system described in this reference a mechanical coupling is caused to slip when the viscosity of a water-ice mixture exceeds a predetermined value, thereby indicating incipient freezing. Upon this slippage, a switch is de-activated, thereby terminating the supply of refrigerant previously passed through coils in indirect heat exchange relation to the liquid being chilled. Although the refrigeration of this liquid is not dependent on the initial temperature thereof, this prior art system utilizes relatively unreliable mechanical elements such as couplings and springs, which are subject to failure, and the drive motor utilized in this system is still subject to line voltage fluctuations which would cause slippage of the coupling element at instances which do not accurately reflect the viscosity, and hence temperature, of the liquid being refrigerated. In addition, although the flow or refrigerant through the coils is terminated, the refrigerant remaining in the coils continues to supply transient refrigeration to the water-ice product which, in turn, tends to reduce the temperature and viscosity below a predetermined level.
Accordingly, a clear need exists for a blender chilling system utilizing CO.sub.2 as a refrigerant and which can be controlled to accurately chill batches of products to desired temperatures, notwithstanding variations in the initial product temperature, composition, or fluctuations in line voltages.